The present invention relates to a method and apparatus for encoding input words consisting of a plurality of information bits so as to produce encoded output symbols. The invention has particular application to the production of encoded output symbols which are to be subject to phase shift key (PSK) modulation. The invention is advantageously employed in generating phase shift key modulated signals for digital television signal transmission.
Recently, a new class of error-control codes, termed xe2x80x9cturbo-codesxe2x80x9d, has been introduced. These codes provide error performance close to the Shannon limit by using an iterative decoding technique that relies upon simple constituent codes. A natural extension to improve the bandwidth efficiency of turbo-codes is to apply them to trellis coded modulation (TCM) and pragmatic trellis coded modulation (PCTM) systems.
In a known modulation system, a turbo-encoder encodes a serial stream of input data bits at a rate R of 1/3 to produce two parity bits in parallel with each input data bit. The information and parity bits are subject to demultiplexing in a demultiplexer so as to form symbols each of which includes a number of information bits and a number of parity bits. The parity bits are punctured in a puncturing module so as to reduce the number of parity bits and thereby increase the data rate of the system. The bits of each symbol are interleaved in an interleaver and passed to a signal mapping module. The output from the mapping module comprises the I and Q components necessary for input to a PSK modulator.
The puncturing operation of the known modulation system depends upon the type of modulation employed and on the desired information rate. In particular, for M-ary modulation with Rxe2x80x2 information bits per symbol, the number of punctured bits is defined as; k=3Rxe2x80x2xe2x88x92logM.
Thus, to generate pragmatic turbo-coded 8-PSK modulation with Rxe2x80x2=2 bits/symbol, the known technique will require the puncturing of k=3 parity bits. For the case of 16 PSK modulation, with Rxe2x80x2=3 bits/symbol, the technique will require the puncturing of k=5 parity bits.
It is known that puncturing provides the desired data rates at the expense of a reduction in the free distance of the code which results in a reduced error performance of the overall system. Furthermore, the need to change the number of punctured bits, depending upon the required data rate, reduces the flexibility of operation.
It is one aim of the present invention to achieve the desired data rate whilst reducing the number of punctured parity bits. A subsidiary aim of the present invention is to increase the flexibility of operation to cope with changes in the desired data rate.
According to the present invention, there is now provided apparatus to encode input digital words of information bits so as to produce output symbols representing the input digital words, the apparatus comprising: a turbo encoder to receive and encode a first portion of each input word so as to generate corresponding encoded and parity bits; a puncturing module to puncture the parity bits once for each encoded bit; and, a mapping circuit to receive an uncoded second portion of each input word, the mapping circuit being adapted to form an output symbol for each input word from the uncoded second portion and from the corresponding encoded and punctured parity bits.
Further according to the present invention, there is provided a method of encoding input digital words of information bits so as to produce output symbols representing the input digital words, the method comprising the steps of: employing a turbo encoder to receive and encode a first portion of each input word so as to generate corresponding encoded and parity bits; puncturing the parity bits once for each encoded bit; and, forming an output symbol for each input word from an uncoded second portion of the input word and from the corresponding encoded and punctured parity bits.